This kind of honeycomb structure has such a structure in which porous honeycomb segments made of silicon carbide or the like are bonded using bonding layer and formed into a predetermined shape with a circular cross-section or the like, and then the periphery thereof is covered with a coating material layer. This honeycomb structure is located in an exhaust system of a diesel engine, and thereby is employed for purifying the exhaust-gas.
Each of the honeycomb segments is separated by porous partition walls, and has a large number of circulation holes extending therethrough in the axial direction. Neighboring circulation holes have one-side ends alternately plugged. Specifically, a circulation hole has an open end on one side, while has a plugged end on the other side. Another circulation holes neighboring to this hole have plugged ends on said one side, while has open ends on the other side.
By adopting such a structure, when exhaust-gas flows into the circulation holes through open ends, the exhaust-gas passes through the porous partition walls to flow out from another circulation holes. Therefore, the partition walls trap the particulates in the exhaust-gas while the exhaust-gas passes through the partition walls, allowing for purification of the exhaust-gas.
In such a honeycomb structure, a structure in which a plurality of honeycomb segments are bonded is adopted, and bonding layers are preferably uniform in thickness. The reason is as follows: Bonding layers with nonuniform thickness cause partially nonuniform thermal conductivity, which tends to cause stress concentration during use at a high temperature, thus deteriorating the durability of the honeycomb structure. In addition, excessively thick bonding layers increase pressure loss in the honeycomb structure, thus lowering efficiency of trapping particulates. On the other hand, excessively thin bonding layers reduce the effect of stress relaxation by the bonding layers.
For the bonding layers with a uniform thickness, conventionally, it has been implemented that, with spacers (space retaining members) of a prescribed thickness made of paper, inorganic materials, ceramics, organic fibers, resin, or the like located between the honeycomb segments, bonding of honeycomb segments allows the bonding layers and spacers to be approximately equal in thickness to each other (Japanese Patent Laid-Open Publication No. 2002-102627).
Bonding of honeycomb segments to each other is performed by fixing the spacers on the honeycomb segment, applying a bonding material to the honeycomb segment with the spacers fixed, and then pressing the honeycomb segments. In the bonding of the honeycomb segments in such a way, achievement of sufficient bonding strength requires application of a pressure of a certain level or more to the honeycomb segments. However, there has been the case that, when the pressure is applied thereto, biting of the spacers into the honeycomb segments, or the like occurs, causing damage to the honeycomb segments.